Recently, personal wireless systems such as a data communication system, used by a portable telephone called a cellular phone, and a wireless LAN built by a personal computer, have been widely used. Reception of digital television broadcasting on the ground wave with a portable personal wireless terminal has been studied in progress. Infrastructure of various wireless systems has been built as discussed above. Amid the foregoing circumstances, personal information terminals including typically a cellular phone are required to make plural and yet different wireless systems available within one information terminal for users' convenience. On the other hand, a cellular system provides a service employing a wider radio frequency bandwidth, e.g. GSM method uses multi-band such as employing additionally 1.8 GHz band, 1.9 GHz band besides the basic bands of 850 MHz band, 900 MHz band. These additional bands are almost twice of the basic ones. The GSM method also uses 450 MHz band which is almost a half of the basic bands. In recent years, another new technique called “software radio” has been studied, i.e. the characteristics of radio device such as frequency, transmission power, and mode of radio wave can be changed with software.
What is necessary for the information terminal to adapt itself to the wireless system, which uses different wireless networks or multi-bands, is to have a radio device that can process signals of wider frequency bands ranging from UHF band to microwave-band. However, an appropriate signal process, such as amplification, selection, and frequency conversion of a high frequency signal of radio frequency, needs a well matching between the elements of a circuit, otherwise the radio device cannot fully exert its performance. To overcome this problem, a conventional technique has generally prepared a plurality of signal processing systems which optimize circuit constants appropriate to the radio frequency bands to be used, and switches over a signal processing system to a proper one when a radio frequency to be used is heavily changed. For instance, Unexamined Japanese Patent Publication No. 2001-186042 discloses a radio device employing GSM method, and this radio device aims to use three radio frequency bands, namely, 900 MHz band, 1.8 GHz band, and 450 MHz band. This conventional radio device has processing systems in parallel for selecting and amplifying the signals of the foregoing three frequency bands.
Another radio device is disclosed in Unexamined Japanese Patent Publication No. 2002-208871, and this one proposes to process radio frequency signals of plural frequency-bands with only one processing system. FIG. 9 shows a structure similar to this radio device. In FIG. 9, matching between antenna 1 and transmit/receive circuit 2 is done by fixed inductor 4 and variable capacitor 3. Choke-coil 5, capacitance 6 and voltage generating circuit 7 form a bias circuit which varies the capacitances of variable capacitor 3 without affecting radio frequency signals. This conventional radio device aims to use both of 800 MHz band and 1.5 GHz band, and the matching in the foregoing two bands is done through varying the capacitance of variable capacitor 3.
In the case of building a radio device applicable to different wireless network systems or wireless systems of multi-bands, the radio device disclosed in Unexamined Japanese Patent Publication No. 2001-186042 needs a plurality of signal processing systems in response to frequency bandwidths. The number of signal processing systems to be mounted in parallel thus increases at a greater number of radio frequency bandwidths to be used. The radio section thus requires a wider space for mounting in the radio device, and a chip area for integrating the functions into a semiconductor increases, which results in an expensive cost.
The radio device disclosed Unexamined Japanese Patent Publication No. 2002-208871 has a variable reactance element formed of only a varactor diode, so that the impedance has a limit to its variable range. It is thus difficult for the radio device to adapt itself to a wide range of frequency bandwidths.